This invention relates generally to a raster output scanning system for producing a high intensity imaging beam which scans across a movable photoconductive member to record electrostatic latent images thereon, and, more particularly, to an apparatus for providing registration of the beam in the process direction of the photoconductive member.
In recent years, laser printers have been increasingly utilized to produce output copies from input video data representing original image information. The printer typically uses a Raster Output Scanner (ROS) to expose the charged portions of the photoconductive member to record an electrostatic latent image thereon. Generally, a ROS has a laser for generating a collimated beam of monochromatic radiation. The laser beam is modulated in conformance with the image information. The modulated beam is reflected through a lens onto a scanning element, typically a rotating polygon having mirrored facets.
The light beam is reflected from a facet and thereafter focused to a xe2x80x9cspotxe2x80x9d on the photosensitive member. The rotation of the polygon causes the spot to scan linearly across the photoconductive member in a fast scan (i.e., line scan) direction. Meanwhile, the photoconductive member is advanced relatively more slowly than the rate of the fast scan in a slow scan (process) direction which is orthogonal to the fast scan direction. In this way, the beam scans the recording medium in a raster scanning pattern. The light beam is intensity-modulated in accordance with art input image serial data stream at a rate such that individual picture elements (xe2x80x9cpixelsxe2x80x9d) of the image represented by the data stream are exposed on the photosensitive medium to form a latent image, which is then developed and transferred to an appropriate image receiving medium such as paper. Color laser printers may operate in either a single pass or multiple pass system.
In a single pass, process color xerographic system, three ROS stations are positioned adjacent to a photoreceptor surface and selectively energized to create successive image exposures, one for each of the three basic colors. A fourth ROS station may be added if black images are to be created as well. In a multiple pass system, each image area on the photoreceptor surface must make at least three revolutions (passes) relative to the transverse scanline formed by the modulated laser beam generated by a ROS system. With either system, each color separation image must be registered to within a 0.1 mm circle or within a tolerance of xc2x10.05 mm. Each color image must be registered in both the photoreceptor process direction (slow scan registration) and in the direction perpendicular to the process direction (referred to as fast scan or transverse registration). Registration in the transverse direction of a single pass ROS system is known in the prior art and a preferred registration technique is disclosed in U.S. Pat. No. 5,237,521 issued on Aug. 17, 1993, assigned to the same assignee as the present invention and hereby incorporated by reference.
In a color printer, the alignment of the lead edge of the color image is made difficult if a Raster Output Scanner (ROS) is used to expose the photoreceptor (PR). Typically as the PR travels into the position where the first scanline is to be imaged, it is sensed by a hole sensor detecting a hole in the belt. It is desired to image the first scanline immediately when this occurs and to repeat this for all four colors to achieve perfect lead edge color registration. However, because of the scanning nature of the ROS imager, the ROS spot, more than likely will not be at the SOS (start of scan) position as the PR hole arrives at the hole sensor. If this is the case, the system must wait until the next scanline crosses the SOS sensor to begin imaging. During this delay, the PR will have traveled and the first scan will be misregistered by a maximum of one full pixel or for a 600 spi single-beam ROS, over 40 microns. A typical prior art registration technique is disclosed in U.S. Pat. No. 5,381,165 showing registration by a feedback loop in which the phase and frequency of SOS signals and a reference signal are compared to produce an error signal representing frequency differences between rotating polygons associated with each Raster Output Scanner in a system of multiple raster output scanners. U.S. Pat. No. 5,808,658 discloses a technique for adjustment of a ROS imager relative to a hole sensor to adjust for registration of an image in wholepixel increments. This technique involves a simplified mathematical procedure not requiring the use of a divide capability in a micro-controller.
In one embodiment, the present invention is directed toward a method and apparatus for sub-scan image registration adjustment of a ROS imager relative to a hole sensor. In this embodiment, the invention is particularly well suited toward single pass multi-ROS imaging systems, allowing precise image-on-image registration by compensating for differences in distance between each ROS imager and its corresponding hole sensor. In another embodiment, the present invention is directed toward a method and apparatus for minimizing an appropriate adjustment during positional rephasing in the rotational position of a polygon within a ROS imager.
According to various embodiments of the invention, a method and a controller for performing the method are provided for calculating image registration in a process direction by minimizing a required phase shift of a rotating polygon in a raster output scanner. The method having the step of determining a requested phase shift in clock cycles of said rotating polygon to align a beam reflected from a first facet of said rotating polygon to a location on a photoconductive surface. If the requested phase shift in clock cycles is greater than a number of clock cycles equal to one-half of a period between the first facet and a neighboring, subsequent second facet, subtract a number of clock cycles equal to a period between the first facet and the second facet from the requested phase shift in clock cycles to determine a required phase shift and decrement a line counter by one. However, if the requested phase shift in clock cycles is less than a negative number of clock cycles equal to one-half of a period between the first facet and the second facet, add a number of clock cycles equal to a period between the first facet and the second facet from the requested phase shift in clock cycles to determine the required phase shift and increment the line counter by one. Although, if the requested phase shift in clock cycles is greater than or equal to a negative number of clock cycles equal to one-half of a period between the first facet and the second facet and is less than or equal to a number of clock cycles equal to one-half of a period between the first facet and the second facet the required phase shift is equated to the requested phase shift. In these embodiments of the invention, the line counter represents a number of scans before a start of image registration on the photoconductive surface.
According to another embodiment of the invention, a raster output scanner imaging apparatus is provided, having a photoconductive surface adapted to move in a process direction relative to a frame, a first rotating polygon rotatably mounted to the frame and having a plurality of facets adapted to reflect a beam onto the photoconductive surface in the form of first scanlines oriented transverse to the process direction, a first sensor corresponding to the first rotating polygon and mounted to the frame and near the photoconductive surface to enable positional information of the photoconductive surface to be detected. Also provided are a second rotating polygon rotatably mounted to the frame and having a plurality of facets adapted to reflect a beam onto the photoconductive surface in the form of second scanlines oriented transverse to the process direction, a second sensor corresponding to the second rotating polygon and mounted to the frame and near the photoconductive surface to enable positional information of the photoconductive surface to be detected and a controller in communication with the first rotating polygon, the first sensor, the second rotating polygon and the second sensor for synchronizing the first rotating polygon and the second polygon to position the first scanlines and the second scanlines at a common location on the photoconductive surface by the use of clock cycle counts. The controller is adapted to adjust a rotational position of at least one of the first rotating polygon and the second rotating polygon based at least on a difference in a first distance between the first rotating polygon and the first sensor and a second distance between the second rotating polygon and the second sensor.
A raster output scanner imaging apparatus is provided in another embodiment of the invention, having a photoconductive surface adapted to move in a process direction relative to a frame, a rotating polygon rotatably mounted to the frame and having a plurality of facets adapted to reflect a beam onto the photoconductive surface in the form of scanlines oriented transverse to the process direction and a sensor corresponding to the rotating polygon and mounted to the frame and near the photoconductive surface to enable positional information of the photoconductive surface to be detected. Also, controller is provided and is in communication with the rotating polygon and the sensor for adjusting a rotational position of the rotating polygon to position the scanlines at a desired location on the photoconductive surface by the use of clock cycle counts based at least on a distance between the rotating polygon and the sensor.
According to another embodiment of the invention, an imaging system for forming multiple superimposed image exposure frames on a photoconductive surface moving in a process direction having a raster output scanner forming a plurality of scanlines in a transverse direction across the width of the member by reflecting modulated beams from a plurality of facets of a rotating polygon, a photoconductive surface indicator for registering images on the photoconductive surface, means for detecting the beginning of a scanline and providing a start of scan (SOS) signal representing the detection, means to detect the relative phase between the lead edge of the start of scan signals and the detection of the indicator including a fast clock timer to provide a phase shift digital count means to detect the rephase adjust required to correct for a distance between the raster output scanner and a sensor corresponding to the raster output scanner, and means to change the speed of the rotating polygon to synchronize the phase of the indicator with the lead edge of the SOS signals and incorporate the rephase adjust.
According to another embodiment, in an imaging system for forming multiple superimposed image exposure frames on a photoconductive surface moving in a process direction, a method of providing scanning speed and phase shift control including the following steps. Forming a plurality of scanlines in a transverse direction across the width of the member by reflecting modulated beams from a plurality of facets of a rotating polygon, sensing scanning speed, providing a signal representing image exposure frame registration, detecting the beginning of a scanline and providing a start of scan (SOS) signal representing the detection, detecting the rephase adjust required to correct for a distance between the rotating polygon and a sensor corresponding to the rotating polygon, determining the relative phase between the start of scan signal and the signal representing image exposure frame registration, converting scanning speed and relative phase into digital signals, and summing the digital signals and subtracting the rephase adjust and inverting the polarity in order to change the speed of the rotating polygon to synchronize the signal representing image exposure frame registration with the SOS signal.